Thallium-201 Cl

Question 1

Physical characteristics

Answer 1

• 68-81 keV characteristic x-ray
• T ½ = 73 hrs
• Allows for delayed imaging

Question 2

Localization

Answer 2

• Active transport via na/k pump
• 3-5% dose localized in the myocardium
• Redistribution

Question 3

Uptake and redistribution

Answer 3

• normal tissue initial uptake
• washout
• recirculation
• re-extraction (initial and washout)

Question 4

Washout

Answer 4

• Depends on perfusion
• ischemic regions indicates slower washout

Question 5

Normal biodistribution

Answer 5

• GI
• Skeletal muscle
• GU
• primarily urinary excretion (CO)

Question 6

stress

Answer 6

• ²⁰¹Tl is injected at peak stress, and images are taken immediately to assess perfusion.
  -Goal: Identify ischemic areas where blood flow is reduced under stress.

Question 7

Redistribution imaging

Answer 7

• Images taken 3-4 hours after stress injection without giving a second dose.
• Why? Thallium behaves like potassium and slowly redistributes into viable myocardial cells.
  Findings:
  ✔ Normal areas: Maintain uptake.
  ✔ Ischemic areas: Show initial defects at stress but improve later.
  ✔ Infarcted areas: Show persistent defects (no redistribution).

Question 8

Rest imaging Performed if defects are seen in stress images.
A second dose of ²⁰¹Tl may be given to distinguish between ischemia vs. infarction.
Goal: Assess myocardial viability—if rest uptake is seen, the tissue is still alive!

Answer 8

• Performed if defects are seen in stress images.
• A second dose of ²⁰¹Tl may be given to distinguish between ischemia vs. infarction.
• Goal: Assess myocardial viability—if rest uptake is seen, the tissue is still alive!

Question 9

Dose

Answer 9

• 111-120 MBq (3-3.5 mCi) for a single injection protocol.
• Lower dose compared to technetium-based tracers (e.g., ⁹⁹ᵐTc-Sestamibi).
• Longer half-life (~73 hours) → Requires longer imaging times.

Question 10

Matrix

Answer 10

• Common SPECT imaging matrix: 64 × 64 or 128 × 128 pixels.
• Higher matrix sizes give better resolution but require longer scan times.

Question 11

SPECT parameters

Answer 11

• Camera Type: Dual- or triple-head gamma camera.
• Collimator: Low-energy, high-resolution (LEHR) or low-energy, all-purpose (LEAP).
• Energy Window: 70-80 keV (primary peak at 72 keV, with secondary peaks at - - 135 keV and 167 keV).
  Acquisition:
  ✔ 180° or 360° orbit
  ✔ Step-and-shoot or continuous mode
  ✔ 20-25 sec per projection
  Processing: Filtered back projection (FBP) or iterative reconstruction (OSEM).

Question 12

Thallium viability

Answer 12

Ischemic cells show…
Decreased uptake
Slower uptake
Slower washout

Question 13

Disadvantages

Answer 13

Increased absorbed dose
Longer imaging acquisition
Reduced count densities
Inferior contrast resolution
 No gating
Attenuation defects